This invention relates to silicon containing resist formulations, and more particularly, to silicon containing photoresist compositions that are suitable for immersion lithography.
The patterning of radiation sensitive polymeric films with high energy radiation flux such as photons, electrons, or ion beams is the principle means of defining high resolution circuitry found in semiconductor devices. The radiation sensitive films, often referred to as photoresists regardless of the radiation source, generally consist of multi-component formulations that are coated onto a desired substrate such as a silicon wafer. The photoresist film is then exposed to radiation. The radiation is most commonly ultraviolet light at wavelengths of 436, 365, 257, 248, 193 or 157 nanometers (nm), or a beam of electrons or ions, or ‘soft’ x-ray radiation, also referred to as extreme ultraviolet (EUV) or x-rays. The radiation is exposed pattern-wise to induce a chemical transformation that renders the solubility of the exposed regions of the film different from that of the unexposed areas. The film is then heated to enhance this chemical transformation. After heating, the film is treated with an appropriate developer, usually a dilute, basic aqueous solution, such as aqueous tetramethylammonium hydroxide (TMAH) to develop the photoresist image on the wafer.
Typical photoresists contain a polymeric component and are generally comprised of a polymeric matrix, a radiation sensitive component, a casting solvent, and other performance enhancing additives. The highest performing photoresists in terms of sensitivity to radiation and resolution capability are the so-called “chemically amplified” photoresists, which provide high resolution, high contrast, and high sensitivity that are not generally provided by other photoresists. Chemically amplified photoresists are based on a catalytic mechanism that allows a relatively large number of chemical events to occur such as, for example, deprotection reactions in the case of positive tone photoresists or crosslinking reactions in the case of negative tone photoresists. The deprotection and/or crosslinking reactions are brought about by the application of a relatively low dose of radiation that induces formation of a catalyst, often a strong acid.
Most of the current positive resist compositions comprise aqueous base soluble functional groups that are sufficiently protected with acid labile groups so that the resist initially will not dissolve in a developer. During exposure to radiation, the photoacid generator (PAG) present in the resist composition produces strong acid, which then catalyzes the removal of the acid labile groups on heating (i.e., upon post exposure baking (PEB)). This process produces aqueous base soluble material in the exposed regions, which is then developed with a basic aqueous developer to produce the images. An image of the projected pattern-wise radiation is formed in the resist film after development, which can then serve as an etch-resistant mask for subsequent pattern transfer steps. The resolution obtained is dependent on the quality of the aerial image and the ability of the resist to maintain that image.
The resolution, R, of an optical projection system such as a lithography stepper is limited by parameters described in Raleigh's equation:R=kλ/NA,where λ represents the wavelength of the light source used in the projection system and NA represents the numerical aperture of the projection optics used. The term “k” represents a factor describing how well a combined lithography system can utilize the theoretical resolution limit in practice and can range from about 0.85 down to about 0.35 for standard exposure systems. The theoretical dimensional limit of equal-sized half-pitch features is one quarter of the wavelength, λ(k=0.25) when NA=1, as the dose applied to the resist is equal to the square of the intensity, and thus the resolution cannot be modulated by any more than λ/4, or a pitch of λ/2. The resolution attainable with each advancing generation of materials has been extended toward these limits through the use of low k techniques and high numerical aperture tools. To obtain images below this feature size using optical lithography, an extension of NA to >1 is needed. Immersion lithography enables lens designs with NA greater than 1.0, thus resulting in an increased resolution of optical scanners. This process requires filling the gap between the last lens element of the exposure tool and the resist-coated substrate with ultrapure water. See A. Hand, “Tricks with Water and Light: 193 nm Extension”, Semiconductor International, Vol. 27, Issue 2, February 2004.
One of the technical challenges facing immersion lithography is the diffusion between the photoresist components and the immersion medium. That is, during the immersion lithographic process, the photoresist components leach into the immersion medium and the immersion medium permeates into the photoresist film. Such diffusion is detrimental to photoresist performance and might result in disastrous lens damage or contamination in a lithography tool that currently costs about 40 million dollars. Therefore, there is a need for a method to prevent interaction between photoresist layers and immersion fluid in an immersion lithography system.
One of the methods that have been quickly adopted by the resist community is the application of topcoat materials on top of the photoresist layer for the purpose of eliminating diffusion of materials from the underlying photoresist layer and to prevent the permeation of the exposure medium into the photoresist film. See, M. Slezak, “Exploring the needs and tradeoffs for immersion resist topcoating”, Solid State Technology, Vol. 47, Issue 7, July 2004. Since water was first proposed as the exposure medium for 193 nm immersion lithography, several topcoat materials have been reported.
As described above, the higher NA allows for improved resolution of smaller feature sizes, however, the higher NA also reduces the depth of focus of aerial images projected onto the resist film. When the depth of focus is relatively shallow, the thickness of the resist film becomes a factor in achieving proper exposure. Thus, thinner resist films may be required for proper exposure at high resolution, but such films often do not yield acceptable overall performance, especially when considering etch requirements for the underlying substrate.
As the resist film is thinned to account for the higher NA, the resist becomes less suitable as an etch mask against later processing of the underlying semiconductor substrate. For example, since the resist film is thin, variation in thickness becomes more significant and may introduce defects into subsequent devices formed on the substrate. Also, micro-channels often form in the upper portions of a resist layer during transfer of the resist image to the substrate by etching. When the resist layer is thin, the micro-channels may extend to the underlying substrate, rendering the resist less effective as a mask.
In addition, the process latitude of many current resists is not sufficient to consistently produce the smaller desired features within specified tolerances. Some of the process parameters where variance may be difficult to avoid include bake time and temperature, exposure time and source output, aerial image focus, and develop time and temperature. The process latitude of a resist is an indication of how wide such variations can be without resulting in a change in the resolution and/or image profile (i.e., size and/or shape of a resist image). That is, if the process latitude is sufficiently wide, then a process parameter may vary, but the variance will not produce a change in the resist image incompatible with specified tolerances.
Another problem that occurs as feature size decreases and pattern density increases is that collapsing of such high aspect ratio features in the resist may occur. Thus, a thinner resist layer may be required to minimize image collapse.
One approach that enables the use of higher NA exposure tools (including 193 nm immersion tools) as well as a thinner photoresist film is multilayer resist processing. One type of multilayer resist processing uses a bilayer (two layer) imaging scheme by first casting a highly energy absorbing underlayer on the semiconductor substrate and then casting a thin, silicon-containing imaging layer (photoresist film) on top of the underlayer. Next, selected portions of the silicon-containing layer are exposed and developed to remove the unexposed portions of a negative photoresist film or the exposed portions of a positive photoresist film. Generally, the underlayer is highly absorbing at the imaging wavelength and is compatible with the imaging layer. Interactions to be considered include adhesion between the two layers, intermixing, and contamination of the imaging layer by the components of the underlayer. Also, the refractive index of the underlayer is matched to the refractive index of the silicon-containing resist layer to avoid degrading the resolution capability of the silicon-containing resist.
As described above, protective topcoats are currently considered vitally important for water immersion lithography. However, the silicon containing resists previously developed are not compatible with current topcoat formulations comprising alcohol solvents (e.g., 4-methyl-2-pentanol). In order to achieve higher resolution by utilizing a bilayer imaging scheme under immersion conditions, there is a need to develop bilayer resists that are compatible with alcohol solvents.